On April 21, ProPublica and PBS’ Frontline investigate American-born terrorist David Coleman Headley, who helped plan the deadly 2008 siege on Mumbai. The film – an updated and expanded version of our earlier investigation, “A Perfect Terrorist” – explores how and why Headley and the other Mumbai plotters slipped past secret electronic surveillance and how Headley planned another Charlie Hebdo-like assault against a Danish newspaper until a tip brought him down.